1. Field of the Invention
The present invention relates to the polishing of hard materials such as metals and the like by means of high frequency vibrational oscillatory vibrations. More particularly, this invention relates to the polishing of the surface of a workpiece by means of a comparatively more vibrationally abradable tool oscillated at frequencies above 1 KHz which, during polishing, develops a form which is a complement of the form of the surface of the workpiece. The oscillatory vibrations of the tool are imparted to a liquid abrasive slurry disposed between the tool and workpiece which abrades the tool to conform to the configuration of the workpiece and at the same time polishes the configuration of the workpiece.
The present invention is particularly adapted to polishing of already formed compound surfaces and complex shapes having fine or intricate detail where a reduction in surface roughness is needed without loss of the existing resolution and detail.
2. Summary of the Prior Art
There are may prior art processes for the polishing of workpiece surfaces, including both traditional and nontraditional processes. Of the non-traditional processes, "superfininshing", "Diprofil" ultrasonic polishing and ultrasonic polishing are perhaps the most common. In the "superfinishing" process, a workpiece is rotated in direct contact with a reciprocating tool, the tool consisting of abrasive particles in a relatively soft binder, so that the rotating workpiece will shape the tool which in turn polishes the workpiece during periods of direct contact therebetween. High frequency vibrations are not involved in this process.
The "Diprofil" polishing process (marketed by the Elgin Corporation, Morton Grove, Ill.) involves the use of a hand held tool having a small abrasive pad at the end of a narrow probe which vibrates at ultrasonic frequencies. In this process, selected surfaces of a workpiece can be manually polished by choosing a tool insert, i.e. pad, which reasonable matches the surface to be posished.
Ultrasonic machining and polishing are well known machining processes whereby the surface of a workpiece is abraded by a grit contained in a slurry circulated between the workpiece surface and a vibrating tool adjacent thereto, with the tool typically vibrating at frequencies above the audible range, i.e. usually within the range of 19,500 to 20,500 cycles per second. The amplitude of vibration is normally less than 0.1 mm (0.004 inch), and typically within the range 0.01 to 0.05 mm (0.0004 to 0.002 inch). Normally, the frequency and amplitude are inversely proportional so that the higher the frequency, the lower the amplitude.
In conventional ultrasonic machining, the abrading tool face is provided with a three-dimensional form, so that a negative complement thereof is machined onto the workpiece surface. Since the tool itself does not contact the workpiece, the actual cutting or abrasion is done by the abrasive particles suspended in the slurry which are caused to impinge against the workpiece surface by the oscillatory vibration of the tool. These particles are driven with a percussive impact against the workpiece surface by the tool, generally vibrating perpendicular to the workpiece surface. The vibrational frequency of the abrasive particles is somewhat less than that of the tool.
It has always been considered as essential in ultrasonic machining that the tool be abraded to the minimum extent possible to thereby extend its service life. Accordingly, tools for this process have typically been made of a material having high strength and good ductility, in order to impart a high degree of impact resistance to the abrading particles and thereby minimize abrasion of the tool itself.
Ultrasonic machining finds particular utility in its ability to work materials which are difficult to abrade such as glass, ceramics, calcined or vitrified refractory materials and hard and/or brittle metals, which are not susceptible to machining by any other traditional technique, or even such nontraditional techniques such as electrical discharge machining, electrochemical machining or the like. Indeed, such materials are more abradable in ultrasonic machining and other comparable processes than are those materials which are easily machined by traditional machining processes. Ultrasonic machining has proven particularly advantageous for reproducing complex shapes which could not be obtained by traditional machining, or even by nontraditional techniques such as electrical discharge machining, electrochemical machining, or the like because of the nature of the materials to be worked.
It is recognized, of course, that ultrasonic machining will impart some abrasive erosion to the tool as well as the workpiece, so that there is an ongoing and increasing loss of fine detail and resolution as the tool is used and worn. Since it is further obvious that the resolution and detail of the image formed into the surface of the workpiece can be no better than that of the tool, it has been considered rather important that the tool material be one that is comparatively tough and ductile, i.e. not readily abradable by the machining action of the vibrating particles, so as that the tool will be abraded to a much lesser degree than the workpiece. For example, tools are commonly made of materials such as titanium, nickel, austenitic stainless steel, cold rolled steel, copper, aluminum and the like which are abraded to a significantly lesser degree than the normally brittle workpiece materials to which the process is appled. Once the tool has been abraded to the degree that the machined surface in the workpiece no longer meets the desired resolution and detail, it is necessary to replace the tool with a new one, or in the alternative redress and reform the image on the tool by such techniques as EDM or the like by which the tool material is more readily machined.
In addition to the above, ultrasonic machining in its normal practice, only abrades areas of the workpiece which are most adjacent to the tool face surfaces, and accordingly, the gap between the tool and workpiece must be very carefully regulated to be as uniform as possible across the entire work surface. Therefore, if ultrasonic machining is to be used on a workpiece that is already formed, or formed in part, as in a polishing operation, it is very important that the tool and workpiece be aligned and registered as accurately as possible. Otherwise, the workpiece will be abraded or polished nonuniformly and possibly even destroyed by the abrasion action. Setting-up the tool and workpiece with the necessarily accurate indexing and registration is a time consuming and laborious procedure as even a very slight misalignment or misregistration can have significant adverse effects on the workpiece being machined or polished.
The foregoing limitations and difficulties have been significant enough to cause operators to choose other machining techniques when the nature of the materials to be worked permit, and has generally required the use of other techniques for polishing operations in particular. Any of the polishing techniques in common use are historically labor intensive, time consuming and expensive operations, and in addition typically require skilled workers and often produce rather inconsistent results. Ultrasonic polishing has been even more demanding in these regards. Polishing by any method requires the removal of a very small amount of workpiece material, and ideally a very uniform removal thereof. Manual polishing, vibratory finishing, buffing, brushing and even extrusion honing cannot remove the workpiece material to the extent of uniformity often desired, particularly in the case of cavities within complex workpiece surfaces. While ultrasonic polishing is capable of removing a very uniform surface layer from the workpiece, this can be done only by assuring a very exacting tool image configuration, by the labor intensive efforts of exact indexing and registration, and the costly frequent tool replacement or redressing.